1. Field of the Invention
The invention relates to an echo canceller for cancelling an echo signal generated in a send path by a signal applied to a receive path. Such an echo canceller comprises a subtraction circuit receiving at its first input a signal derived from the signal entering the send path and at its second input the output signal of a transversal filter receiving a signal derived from the signal applied to the receive path. The echo canceller is provided with means for adjusting the filter coefficients so as to cancel the component due to the echo signal in the output signal of the subtraction circuit.
2. The Prior Art
An echo signal may be produced for example by imperfections in a two-wire to four-wire coupling circuit in a telephone exchange for coupling a send path and a receive path together forming a four-wire circuit to a telephone subscriber's line. Another possible origin of an echo signal is to be found in an acoustic coupling between the microphone and the loudspeaker of a telephone set. Whatever its origin, the echo signal produced in the send path may be considered as the response of an echo path to the signal transmitted by the receive path. This parasitic echo signal may interfere with the reception at the location of a distant subscriber, of the useful signal transmitted by the send path. A known method of cancelling the echo is to use a transversal filter connected to the send path and whose coefficients are adjusted in an adjusting circuit in such a way that the response of the filter is as close as possible to that of the echo path, such that by subtraction of the filter output signal from the signal entering the receive path the echo signal is practically cancelled at the output of the receive path.
A limitation to the use of echo cancellers in telephone circuits lies in the complexity of the calculations to be made. It is known that in the transversal filter of the echo canceller it is necessary to perform at each sampling instant a summation of a certain number of samples of the input signal, previously multiplied by coefficients to be adjusted so as to be as close as possible to samples of the impulse response of the echo path. The coefficients of the filter are generally adjusted in accordance with the gradient algorithm, to minimize the mean square value of the signal at the output of the subtraction circuit. This amounts to adjusting coefficients by successive iterations in accordance with an iteration formula described for example in an article by Tanaka et al. entitled "A Multi-channel Echo Canceller System", which appeared in NEC Research and Development, No. 49, April 1978, pages 58-64. According to this algorithm, the value used to adjust each coefficient at each iteration results from multiplication of an error signal sample, constituted by the output signal of the subtraction circuit, by one of the samples of the input signal stored in the filter, and from a division of the product thus formed by the sum of the squares of a certain number of samples of the filter input signal. The filter coefficients have to be determined with precision. Thus the multiplications to be made in the filter and in its adjusting circuit are complex and costly operations, if the signal applied to the filter input and coming from the reception path is PCM-encoded in accordance with a linear law or even in accordance with a pseudo-logarithmic law (A-law or .mu.-law) as practised at the present time: on this subject, see for example the echo canceller described. In U.S. Pat. No. 4,064,379, in which this patent the analog signal transmitted by the receive path is converted into a pseudo-logarithmic PCM-signal according to the A-law for processing in the echo canceller.
The object of the present invention is to provide an echo canceller in which the calculations to be made in the transversal filter and in its adjusting circuit are considerably simplified compared with those required in known echo cancellers.
In accordance with the invention, an echo canceller intended for cancelling an echo signal generated in a send path by a signal applied to a receive path is characterized in that it comprises:
a delta encoder for encoding, at a sampling rate 1/T, the signal applied to the receive path and for supplying to the input of the transversal filter the bits of the delta-encoded signal,
a difference-forming circuit for forming, at said sampling rate 1/T, the difference signal between the values of two consecutive samples of the signal entering the send path and for supplying this difference signal to the first input of the subtraction circuit, and
an integrating circuit for integrating the output signal of the subtraction circuit and for applying the integrated signal to the output of the send path.
The output signal of the receive path and applied to the echo path may be the signal applied to this receive path, but preferably this output signal will be supplied by the local decoder present in the delta encoder.
In that case it is advantageous if the signal leaving the receive path is applied to a filter that cuts off the frequencies higher than the upper frequency of the signal band applied to the receive path.
The coefficients of the transversal filter may be adjusted by successive iterations in accordance with the gradient algorithm. In that case the coefficients adjusting circuit of the transversal filter is provided with means for adjusting the coefficients h.sub.i by successive iterations at said sampling rate 1/T, in accordance with the iteration formula: ##EQU1## where: i is an integer going from 0 to p, pT being the duration of the impulse response of the echo path,
h.sub.i (n) are the filter coefficients to be adjusted at the instant nT, PA1 h.sub.i (n+1) are the adjusted filter coefficients utilisable at the instant (n+1)T, PA1 e.sub.n is the value of the signal supplied by the subtraction circuit at the instant nT, and PA1 .DELTA.F.sub.n-i are the values of the bits of the delta-encoded signal, stored in the transversal filter and entered into this filter at the instants (n-i)T. PA1 d.sub.n-j are the values of the difference signal supplied by the difference-forming circuit at the instants (n-j)T, and PA1 .DELTA.F.sub.n-j-i are the values of the bits of the delta-encoded signal stored in the filter and entered into the filter at the instants (n-j-i)T.
If a variable-step delta encoder is used this may be taken into account by inserting in the output of the transversal filter a multiplier for multiplying by the encoding step while at the same time providing the coefficients adjusting circuit of the transversal filter with means for multiplying the coefficients, formed in accordance with the above-mentioned iteration formula, by ##EQU2## where .delta.f.sub.n, .delta.f.sub.n+1, .delta.f.sub.n-i, .delta.f.sub.n-i+1 are the respective values of the encoding steps at the instants nT, (n+1)T, (n-i)T, (n-i+1)T, the coefficients h.sub.i (n+1) resulting from this multiplication being the adjusted coefficients usable at the instant (n+1)T.
The coefficients of the transversal filter may also be calculated using a "statistical" method. In this case the coefficients adjusting circuit of the transversal filter is provided with means for calculating the coefficients h.sub.i of the filter using the expression ##EQU3## where: q is an integer such that q.angle.p,